From e9a6944ae559c0a4a1bae5fae0dd0040cb11fc31 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Tue, 21 Jun 2016 22:54:34 -0400
Subject: [PATCH] Fix Double World Add issues

Vanilla will double add Spider Jockeys to the world, so ignore already added.

Also add debug if something else tries to, and abort before world gets bad state

diff --git a/src/main/java/net/minecraft/server/ChunkRegionLoader.java b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
index 4a0f3989e6..695044b1b1 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -1052,7 +1052,7 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
     }
 
     public static void a(Entity entity, GeneratorAccess generatoraccess, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason reason) {
-        if (generatoraccess.addEntity(entity, reason) && entity.isVehicle()) {
+        if (!entity.valid && generatoraccess.addEntity(entity, reason) && entity.isVehicle()) { // Paper
             // CraftBukkit end
             Iterator iterator = entity.bP().iterator();
 
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index d31101861c..a81c7dfa2e 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -993,6 +993,7 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
     public boolean addEntity(Entity entity, SpawnReason spawnReason) { // Changed signature, added SpawnReason
         org.spigotmc.AsyncCatcher.catchOp( "entity add"); // Spigot
         if (entity == null) return false;
+        if (entity.valid) { MinecraftServer.LOGGER.error("Attempted Double World add on " + entity, new Throwable()); return true; } // Paper
 
         org.bukkit.event.Cancellable event = null;
         if (entity instanceof EntityLiving && !(entity instanceof EntityPlayer)) {
-- 
2.19.0